1. Field of the Invention
The present invention relates to a method of producing a slip casting mold which is used to fabricating formed articles by casting from a slip which contains, for example, ceramic powders, metal powders and carbon powders, and also to a slip casting method using the mold. More particularly, the present invention is concerned with a method of producing a slip casting mold which is used in molding of articles which necessitate cores and mold parts having such complicated configurations that they cannot be extracted due to inverse tapers, as well as to a slip casing method making use of such a casting mold.
2. Description of the Prior Art
In casting articles having complicated outer and inner configurations from a mold, it has been a common measure to form the mold by assembling a multiplicity of main mold parts and cores. This method, however, requires a number of steps for the preparation of many mold parts and cores. In addition, the quality of the products tends to be impaired due to the presence of burrs.
A method for overcoming these problems is disclosed in the specification of British Patent No. 1482436. According to this method, mold parts having complicated configurations are formed from an organic material, while other mold parts of comparatively simple configurations are formed from gypsum. As a slip is poured into a resultant mold, the gypsum absorbs the water contained in the slip to solidify the slip. Thereafter, the organic material is dissolved by a solvent, whereby a wet shaped body generally referred to as a "green body" is obtained.
This method, however, suffers from the following disadvantages.
(i) Dissolved residue of organic material inevitably remains locally on the surface of the green body. The more complicated the article configuration, the more difficult the removal of the dissolved residue. The dissolved residue remaining on the product surface due to incomplete removal or due to penetration into the green body seriously deteriorates the quality of the product. PA1 (ii) An impractically long period of time is required for solidification of the green body, because only a limited portion of the mold can absorb water. PA1 (1) preparing a pattern; PA1 (2) providing a mold (forming a cavity); PA1 (3) casting a slip; PA1 (4) solidifying the slip (a liquid medium contained in the slip is absorbed in mold walls so that a green body is obtained; PA1 (5) removing the mold (the green body is obtained); and PA1 (6) sintering the green body. PA1 (a) Unless assembling many mold portions (main molds and cores), it is impossible to provide a mold cavity of a complicated shape (regarding the above-described mold-forming step 2); PA1 (b) A green body is apt to suffer stress from both the rigid cores and main molds with the result that cracks are apt to occur in the green body, during the solidification of the slip and during the shrinkage of the green body caused due to the removing of water and due to drying of the green body (the step 4 of solidifying the slip); and PA1 (c) In order to remove a mold without causing flaws on the green body after the green body is prepared by the solidifying of the slip, the mold must be divided into many mold portions because the strength of the green body is very low in comparison with the mold, that is, the strength of the green body is not more than one fifth that of the mold (the step 5 of removing the mold).
The specification of U.S. Pat. No. 2,830,343 discloses a method in which a pattern made of foamed polystyrene is embedded in molding sand. As a molten metal is poured into the mold as it coexists with the pattern, the pattern is melted by the heat of the molten metal, so that the space which has been occupied by the pattern is replaced by the molten metal. This method is generally called a full-mold casting method. Considering that the removal of the pattern and the substitution by the molten metal have to be conducted simultaneously, it would be very difficult to apply the full-mold casting method to slip casting.
Thus, the method disclosed in the specification of British Patent No. 1482436 is disadvantageous in that the dissolved residue of organic material remains on the surface of the green body. As explained before, the more complicated the article configuration, the more difficult the removal of the residue. Any residue remaining on the surface of the green body due to incomplete removal or any fraction of the residue penetrated into the green body impairs the quality of the product. Furthermore, a high degree of skill and experience, as well as a number of steps, are required for the assembly of the mold from many mold parts and cores.
On the other hand, the application of the full-mold method disclosed in the specification of U.S. Pat. No. 2,830,343 to slip casting is considered to be difficult in view of the fact that the removal of the pattern and the substitution by the molten metal have to be conducted simultaneously.